Throw exception when adding duplicate key from fast to safe

[mdedetrich]

If it doesn't already do so, we should throw an exception if we are adding a duplicate key when using toSafe on a fast.JObject. This may have a performance impact however, so we should have a look at this carefully.

I believe that currently, it will silently ignore the duplicate key, which isn't behaviour that we want (I have to check how the builder works in this area)

Current code is located at https://github.com/json4s/json4s-ast/blob/master/jvm/src/main/scala/org/json4s/ast/fast/JValue.scala#L87-L101

[mdedetrich]

Relevant issue also at #8